1. Field of the Invention
The present invention relates to an electrostatic imaging system and, more specifically, to an apparatus for transferring a toner image from an image-forming medium to a receiving material via an intermediate transfer medium and for cleaning the intermediate transfer medium.
2. Discussion of Related Art
U.S. Pat. No. 4,607,947 describes a contact fixing apparatus in which a toner image is transferred from an image-forming medium to a heated intermediate. In a fixing zone in which the intermediate is in contact with a pressure applying means, the toner image is then transferred to and simultaneously fixed on a receiving material transported through the fixing zone. However, impurities may also be transferred at the same time from the receiving material to the intermediate. Residues of toner material may also be left as impurities on the intermediate because of incomplete transfer of the toner image to the receiving material. If such impurities remain on the intermediate they may be transferred to the image-forming medium in the first transfer zone. This results in disturbance of the image formation and hence ultimately image errors in the copy on the receiving material.
Various cleaning means have been proposed to remove these impurities from the intermediate before reaching the first transfer zone. For example, U.S. Pat. No. 4,607,947 discloses a cleaning means having a cleaning surface to which toner has better adhesion than to the intermediate. A cleaning means of this kind operates satisfactorily for removing high-melting temperature impurities, such as toner residues. This cleaning means can also remove paper dust from the intermediate, but it has been found in practice that low-melting impurities from receiving materials, such as wax-like compounds, plasticizers, anti-foaming agents, plastic fillers which occur in receiving papers, and dust particles from plastic receiving materials and the like, are removed only partially, if at all, from the intermediate with the known cleaning means. After deposition on the intermediate in the second transfer zone, these impurities can also then be transferred to the image-forming medium in the first transfer zone, resulting in disturbance of the image formation and hence ultimately image errors in the copy on the receiving material. This necessitates regular and premature replacement of the intermediate and image-forming medium, and this involves high maintenance costs and equipment stoppage. For example, it has been found that the "alkaline" receiving papers increasingly used and based, inter alia, on cellulose, chalk and sizing agents such as alkyl ketone dimers, are a significant source of such impurity. Receiving papers of this kind are currently used because of the lower costs and better durability compared with the "acid" receiving papers based, inter alia, on cellulose, clay and modified or unmodified rosins. It has now been found that when such receiving papers are used, reaction products from these binders are deposited from these papers on the intermediate. These reaction products are then transferred to the image-forming medium, resulting in image disturbance.